U.S. PATENT DOCUMENTS
Not Applicable
Power and power density are among the most important characteristics of laser beams to be measured and controlled for essentially all laser applications. Present technologies provide a number of solutions, however, they are not suitable for high power laser beams. Citing U.S. Pat. No. 4,035,088 to A. H. Jenkins and J. J. Wachs, which is incorporated by reference, xe2x80x9c. . . the highly intense laser beam oversaturated and destroyed known energy detectors and power meters with the exception of large heat sink spherical calorimeter, which totally blocked the beam.xe2x80x9d Blocking the beam does not allow in-line monitoring of the beam and, due to large temperature increase, has other disadvantages too such as the impossibility of measuring relatively fast variations in the beam power during operation of the laser.
A laser beam sampling technique that overcomes some of these problems for power measurement is described in the U.S. Pat. Nos. 4,260,255 and 4,035,088 to Wachs and Jenkins. In these inventions, an apparatus for sampling and measuring power of a high energy laser beam without blocking or unduly perturbing the beam is realized with the aid of highly reflective ribs having knife edges that are making an angle with respect to the propagation direction of the beam to reflect part of it out of the main beam for energy measurement.
Even more difficult is characterization of the profile of high power laser beams and measuring its radius which would allow determination of the power density of radiation. There were many attempts of overcoming the problems outlined above. One approach relies on registering the changes in the spatial profile of the beam propagated through a transparent nonlinear optical material. Such an apparatus for power density measurement of electromagnetic radiation is suggested in U.S. Pat. No. 5,621,525 to Tabirian et al. which is incorporated by reference. For high power industrial or defense laser beams, even residual absorption in the nonlinear optical material may, however, lead to saturation and destruction of the nonlinear optical element constituting the sensor head of the laser beam measurement device.
Many present-day devices for laser beam profile measurements use knife-edges, blades and slits for scanning across the beam. The transmission of the beam past the knife-edge (or any other aperture) is then monitored for characterization of the power density distribution across the beam. Several patents, such as U.S. Pat. No. 5,100,231 to Sasnett et al, U.S. Pat. No. 4,160,598 to Firester et al, describing such approaches are incorporated by references. The disadvantage of laser beam profiling devices that are using knife-edges, slits, pinholes, and other apertures is that they are blocking the beam. Consequently, in-line profiling of the laser beam is not possible, and the high-energy laser beams can damage the apertures.
Though it is principally possible to sample a small portion of a high power laser beam and then measure it with imaging CCD, such technique is also not satisfactory due to the absence of sufficiently high damage threshold, large area and inexpensive sampling materials. As an example, a diffractive sampling element is described in U.S. Pat. No. 5,323,267 to Galarneau et al. The diffractive optical element is made of optical materials such as glass or crystals. These materials are still rather vulnerable to high power laser radiation. Internal or laser-induced inhomogeneities in such materials distort the beam. Their size is technologically limited to 1-2 inches in diameter. Moreover, the optical materials that shall be used for infrared radiation (such as Zinc Selenide and Calcium Fluoride) are expensive.
Thus, present devices for measurement of laser beam diameter and characterizing of its profile are not satisfactory for high power laser beams; they are based on hypersensitive imaging techniques (CCD), use expensive optical materials vulnerable to radiation, are based on optical components with strict tolerances on their homogeneity and thickness, and can not be carried out without blocking the beam.
The first objective of the present invention is to provide a technique for non-obstructive in-line measurement of the diameter of laser beams, particularly, those of high power infrared beams.
The second objective of this invention is to provide means for in-line determination of a laser beam quality, particularly for high power and infrared beams.
The third objective of this invention is to provide means for in-line characterization of spatial profile of laser beams.
The fourth objective of this invention is to provide means for non-obstructive in-line characterization of energy parameters of laser beams.
The invention includes driving a thin metallic wire, or a thread made of other material, across the beam and registering the portion of the beam scattered from the thread with the aid of a detector which, in general, may include a lens that is collecting said scattered light. The speed of scan of the wire across the beam is set by the given driving technique or it is independently measured by using a reference wire at a pre-established fixed distance from the first one. The time delay between the signals received from both of the wires allows independent determination of the speed of scan of wires across the beam which is used then for calculating the spatial width of the beam from the measured temporal width of the signal of radiation scattered from the wires.